Distribution and quantification of FGFR1/2⁺ mesenchymal cells after sublethal TBI. (A) Ten days after irradiation (10 DPI) at 9 Gy, the periosteum showed evidence of intense remodeling and FGFR2⁺ cells were observed in vascularized PN⁺ canals on the cortical bone surface (see also supplemental Figure 1B). Scale bar indicates 50 μm. (B) Flow cytometric quantification of FGFR1⁺ and FGFR2⁺ mesenchymal cells in whole bones 10 days after sublethal irradiation. (C) Cells in the R2 region (Lin⁻/CD45⁻/FGFR1 or 2⁺) were compared with total cell numbers obtained from 5 nonirradiated control mice and 10 irradiated test mice. The frequency (%) of FGFR1/2⁺ cells was increased after irradiation, but their absolute number was relatively stable, suggesting that these cells are radio-resistant. APC indicates allophycocyanin.